A lake is the opposite of an island; it’s a body of water completely surrounded by land. However, widened portions of rivers and coastal bodies of water that are connected to the sea are sometimes called lakes, also. For example, Louisiana’s Lake Pontchartrain was formed by the Mississippi River. (Fronting New Orleans, it’s also very near the sea.) Venezuela’s “Lake” Maracaibo is sometimes described as South America’s largest lake, though it’s actually an extension of the Caribbean Sea.

The study of lakes is called limnology. Lacustrine is an adjective that describes things related to lakes. Thus, animals adapted to a lacustrine habitat live in lakes.

The primary source of lake water is precipitation (rain and snow). In fact, precipitation that falls on some 40% of Earth’s land surface flows into lakes. Rain may fall directly into lake basins, or it may flow into lakes as runoff from surrounding higher ground or through underground springs.

Most water leaves lakes through transportation or overflow. Lake Superior has a water retention/replacement time of 191 years. In other words, water that enters the lake, on average, stays in the lake for nearly 200 years.

Water temperature varies within a lake and between seasons. The surfaces of lakes located at high latitudes or altitudes may freeze in winter. Ponds are generally small, shallow lakes that have a more even temperature.

Most lakes are “freshwater;” that is, they are not salty like the sea. Salt and bitter lakes are most common in dry areas where water evaporates faster than it can be replaced, leaving high amounts of mineral salts. The most famous such lakes are Asia’s Caspian Sea, the Dead Sea and Utah’s Great Salt Lake. Tanzania’s Lake Natron is one of several “soda lakes” that lie in Africa’s Great Rift Valley. Their salty condition is caused by volcanic gases or fluids.

Lakes are important sources of drinking water, electricity and water for irrigation. Some lakes also support commercial fishing or are valued for recreation. North America’s Great Lakes serve as an important transportation system.

Lake Life

Lakes have special communities of flora (plants) and fauna (animals) depending on such things as the size and shape of the lake, surrounding rocks and soil and the local climate. Living things are usually found in one of three lake zones. The shallower edges of a lake are called the littoral zone. Marsh vegetation such as cattails and water lilies often grow here. Farther from shore is the limnetic zone. The only plants that are likely to be found here are floating plants, unless the lake is very shallow. Deep lakes also have a profundal zone where little light is found and oxygen is scarce. There is less life in this region.

How Are Lakes Formed?

You need just two things to form a lake: A source of water, and a basin, or depression, where it can collect.

More of the world’s lakes were formed by glaciers than by any other method. During the Ice Age, much of the Northern Hemisphere was covered with great glaciers that cut or enlarged depressions in the earth which later filled with water. These are called glacial lakes.

North America’s Great Lakes are glacial lakes that were carved out of bedrock. Other glacial lakes formed in depressions in the rubble, or “drift,“ deposited by glaciers.

Glacial lakes vary in size from the enormous Great Lakes to the “prairie potholes” strung across North Dakota and neighboring states and provinces. Scotland’s “lochs,“ including the famous Loch Ness, are glacial lakes.

Many alpine lakes—lakes found high on mountain slopes—are glacial lakes; they’re often called tarns in Europe.

Lakes and ponds are also very abundant in many tundra regions, where frozen ground called permafrost keeps water from sinking into the earth. Though Minnesota is nicknamed the Land of 10,000 Lakes, there are far more than that scattered across northern North America and Eurasia. Canada boasts 50% of the world’s lakes!

Barrier lakes are formed by landslides or glacial drift. Such earthen walls sometimes dam former river valleys to form lakes.

Some enormous lakes even formed far away from the great glaciers during the Ice Age. The climate in the Great Basin was much cooler and wetter than it is today, and vast lakes covered much of what are now Nevada and Utah. As the climate became more arid (dry), the lakes began to shrink. As Lake Bonneville shrank, salt was greatly concentrated in the remaining water. Today, we know it as Utah’s Great Salt lake.

Lakes are also common along slow-moving rivers and in low areas near the sea. Portions of meandering rivers that are separated from the main river become isolated lakes known as oxbows.

Tectonic lakes occur in natural fissures. An example is Lake Tanganyika, which fills a depression in Africa’s Great Rift Valley—a region where the continent is being pulled apart.

Karst lakes form in areas where limestone is dissolved to form cavities and depressions; they are common in Florida and Appalachia. Groundwater may even dissolve limestone to form underground lakes.

Lakes can also form in volcanic calderas. Among the most famous crater lakes is Oregon’s Crater Lake. It’s fabulously clear, deep-blue waters are 1,932 feet (589 meters) deep!

Ocean currents may deposit sediments along a shoreline, cutting off bays which then become coastal lagoons.

Many artificial lakes have been created as sources of hydroelectricity, water for irrigation and other uses.

How Do Lakes Die?

Lakes are like animals in that they are born, grow, then die. Some lakes disappear when their source of water is less. An example is Utah’s Great Salt Lake, which was much larger during the wetter, cooler Ice Age.

Lakes are also erased through a process called eutrophication, in which a lake’s basin is filled with sediments. (These include organic sediments, or dead plants and animals.) Just imagine shoveling sand into a swimming pool, forcing the water out. Over time, a lake becomes shallower until it may become a swamp or marsh—a watery grassland or wooded area. Next, it transforms into a bog, with little or no standing water. Finally, it may become a meadow.

But new lakes are also constantly being born.

Lake Statistics & Records

It is estimated that more than 80% of the world’s freshwater occurs in the form of groundwater or ice. Thus, 20% occurs largely as rivers and lakes.

About 80% of the freshwater that occurs in lakes (about 30,000 cubic miles, or 125,000 cubic kilometers) occurs in as few as the 40 largest lakes.

However, the world’s largest lake is a SALTWATER body called the Caspian Sea. This Asian lake has a surface area of about 150,000 square miles, is about 750 miles long and has a maximum depth of about 3,100 feet.

Lake Superior has a greater surface area than any other freshwater lake (about the size of Maine, or one fifth the size of the Caspian Sea). It is the largest of five “Great Lakes” that cover about 95,000 square miles (245,000 square kilometers), forming a great inland waterway shared by the United States and Canada.

Lake Superior contains 3,000,000,000,000,000 gallons of water. That’s three quadrillion gallons, or 11,400,000,000,000,000 liters. It’s also 10% of the world’s fresh surface water and enough to submerge North America and South America under one foot of water.

All five Great Lakes combined contain about 5,500 cubic miles (22,900 cubic kilometers) of water.

Yet Asia’s Lake Baikal contains about the same amount of water as all five Great Lakes combined. About 20% of Earth’s freshwater (other than groundwater and ice) is found in Lake Baikal, which is deeper than any other lake, reaching depths of more than a mile.

Lake Baikal is known for its somewhat unusual process of self-purification, which makes it one of the world’s clearest lakes. (50 meters) Baikal is also home to more endemic species (species found only in Baikal) than any other lake, including the world’s only freshwater seal. Thermal springs pump hot, oxygenated water into Lake Baikal, giving it “living water” as opposed to all other deep lakes, whose lower depths are dead. Clearly, this is no ordinary lake!

At 12,500 feet (3,810 meters) above sea level, Peru’s Lake Titicaca is the world’s highest lake that’s navigable by large vessels. Outside of Alaska, there aren’t many mountain peaks in the United States that high!

The lowest lake in the world is the salty Dead Sea, which lies in a sunken rift in Israel and Jordan with a surface 1,316 feet (401 meters) below sea level.

Of the 25 largest lakes, North America boasts the most (10). Nine of these are in Canada, which boasts more than any other nation. Seven are in Asia (including the two biggest), none in Europe. Two are in South America, one in Australia.

All of the six giant lakes located in the United States are products of the Ice Age. The five Great Lakes occupy basins carved by Ice Age glaciers, while Utah’s Great Salt Lake is a souvenir of a much larger lake that was nurtured by a cooler, wetter climate during the Ice Age.